The present invention relates to a lock control method for on-board terminal equipment of a mobile telecommunications system, particularly a car telephone system.
A car telephone system or similar mobile telecommunications system extensively used today includes on-board terminal equipment each having a controller which is implemented as a CPU, for example. Since this kind of mobile telecommunications system uses electromagnetic waves which is a finite resource, it costs more free than ordinary wired telephone systems. For this reason, the on-board terminal equipment has customarily been provided with a locking function for preventing persons other than the user and an authorized person from originating a call thereon. Specifically, with the locking function, the equipment can be readily locked by anyone, but cannot be unlocked unless a single unlock code of which only the user and authorized person are well aware is entered. The user or the authorized person, therefore, may lock the equipment whenever he or she leaves the vehicle or after using the equipment. This is successful in preventing the other person who, for example, has stolen the vehicle from using the equipment.
The problem with the conventional locking function is that even an unauthorized person who does not know the unlock code may, by chance, unlock the terminal equipment by entering some different codes one after another. Although such an occurrence may be coped with by increasing the number of figures of the unlock code, this will render the operations of the equipment troublesome.